The present invention relates to solar energy systems, and in particular relates to solar power propulsion systems which use thermal energy for propelling and powering a spacecraft.
Thruster devices are often used in the space industry, particularly for low power applications such as orbital positioning of a spacecraft. In particular, a thruster device, such as a solar thruster, may be used for correcting or maintaining the orbit of a satellite. Another application of thruster devices is to change or alter the orbital path of a satellite in order to avoid detection by other satellites or ground tracking devices.
Conventional solar thruster devices typically comprise a heat exchanger made of a material having a high coefficient of thermal conductivity, such as rhenium, tungsten, or halfnium carbide. The heat exchanger typically includes or defines a flow passage through which a working fluid or propellant is directed such that the solar energy incident upon the heat exchanger is transferred to the propellant. The high temperature propellant is then expelled from the thruster device by expansion through a nozzle, thereby producing thrust. The thrust is used by the satellite to maneuver between orbits or maintain a particular location.
The solar energy used by the thruster is gathered by the heat exchanger by means of a primary concentrator in conjunction with a secondary concentrator. The primary concentrator receives solar energy directly or by reflection and transfers this energy to the secondary concentrator which, in turn, directs the solar energy to the heat exchanger of the thruster device. The energy absorbed by the heat exchanger is then transferred to the propellant within the flow passage. The role of the flow passage is to provide a relatively long flow path along which the propellant can be heated and to direct the propellant to the nozzle. The most effective approach to achieving high thruster performance is to attain a high propellant temperature. The key to achieving this desirable high propellant temperature is dependent on the maximum allowable heat exchanger temperature as well as its thermoconductivity and other properties.
While the use of thruster devices is known, conventional thruster devices can be relatively difficult to manufacture. In particular, a conventional thruster design typically includes a flow passage formed by winding sections of three-foot metallic tubes around a cylindrical mandrel. The cylindrically wound tubes provide a large surface area for transferring solar energy to the propellant. The tubes are typically comprised of a refractory metal capable of withstanding high temperatures, such as rhenium. However, as the tubes are wound around the mandrel, the metal tubes work harden and are very difficult to form to the desired shape. In addition, the tube sections must be welded together prior to winding around the mandrel. Thus, this design requires a tedious, time-consuming and costly fabrication process.
Another conventional thruster device developed by Boeing comprises a cylindrical capsule-like body portion having a nozzle at one end. In contrast to winding a tube around a mandrel to form the flow passage, a series of spirally wound grooves are formed in an inner liner of the body portion, and an outer layer of the body portion is slid over the inner liner to form a spiral channel that directs propellant to the nozzle. This design, however, requires that the diameteral clearance between the inner and outer liners be closely matched to avoid chafing between the liners upon assembly. In particular, chafing can damage the seal between the grooves forming the channel, which can have a deleterious effect on the flow of propellant and result in a loss of thrust.
In yet another design, a rhenium foam is positioned between two rhenium sheets to form a flow passage. However, this thruster design has not been proven experimentally.
Thus, there is a need for a thruster device that offers the same or improved performance relative to conventional thruster devices with reduced fabrication costs. In particular, it would be desirable to fabricate the thruster device without having to wind tubes around a mandrel to form the flow passage. There is also a need for providing a thruster device that can be easily assembled without damaging the flow passage.
These and other needs are provided, according to the present invention, by a thruster device having frustoconical inner and outer layers and a flow passage interposed between the two layers. The flow passage can be defined by the inner layer, outer layer, or both layers. As such, the thruster device of the present invention provides a flow passage formed without having to wind a tube around a mandrel, thereby simplifying the fabrication process. Advantageously, the frustoconical configuration of the inner and outer layers of the thruster device of the present invention allows for easy assembly of the thruster device with minimal risk of damaging the flow passage due to chafing between the inner and outer layers. Thus, the resulting thruster device can typically be fabricated relatively inexpensively.
In particular, the thruster device of the present invention advantageously comprises a frustoconical inner layer having inner and outer surfaces and opposed ends. A frustoconical outer layer surrounds the inner layer and means for defining a spiral flow passage are disposed between the inner and outer layers. Preferably, the inner and outer layers cooperate to define the spiral flow passage themselves. In one advantageous embodiment, the inner and outer layers are formed of at least one of rhenium and hafnium carbide in order to have good heat exchange properties. An outlet, such as a nozzle, is attached to one of the opposed ends of the inner and outer layers such that the outlet is in fluid communication with the spiral flow passage. In one embodiment, the thruster device also includes an end flow cap connected to the same end of the inner and outer layers for establishing fluid communication between the spiral flow passage and the outlet. The end flow cap includes at least one radial channel for directing the propellant from the spiral flow passage to the outlet.
A method of manufacturing a thruster device is also provided. In particular, the method comprises forming a spiral flow passage in at least one of the frustoconical inner and outer layers, and nesting the inner layer within the outer layer such that the spiral flow passage is defined between the inner and outer layers. The method also includes attaching an outlet, such as a nozzle, to at least one of the inner and outer layers such that the outlet is in fluid communication with the spiral flow passage. In one embodiment, the method also includes attaching an end flow cap between at least one of the inner and outer layers and the outlet for directing propellant to the outlet.
The spiral flow passage is formed by covering portions of one of the inner and outer layers with photoresist having a spiral pattern and then etching the uncovered portions. In one embodiment, the spiral flow passage can be formed by applying photoresist to one of the inner and outer layers and then exposing the photoresist to define a spiral configuration. The flow passage can then be finished by chemically etching the portions of the inner or outer layer from which the photoresist has been ablated. Thus, the flow passage can be formed to within precise tolerances.
Advantageously, the present invention provides a thruster device and associated methods that overcomes the shortcomings of conventional thrusters discussed above. In particular, the thruster device of the present invention provides a flow passage that is preferably defined by the inner and/or outer layers, instead of requiring a tube to be wound around a mandrel as in some conventional thruster devices. Furthermore, the thruster device of the present invention is easy to assemble, since the frustoconical configuration of the inner and outer layers do not abrade against one another as the outer layer is positioned over the inner layer. As such, the thruster device of the present invention is improved in both cost and performance. In particular, the design of the thruster device significantly simplifies the fabrication process compared to conventional thrusters by reducing the number of parts required to form the flow passage and eliminating the costly and time consuming process of welding and winding tubes about a mandrel to form the flow passage. In addition, the design of the thruster device reduces the likelihood of damaging the flow passage during assembly by reducing, if not eliminating, abrading and other deleterious contact between the inner and outer layers as the inner layer is inserted into the outer layer. Thus, the thruster device of the present invention has reduced fabrication costs. As a result, the propellant can be directed to the outlet in a more efficient manner, which results in higher performance of the thruster device. Advantageously, the improved performance of the thruster device allows for greater payload capacity along with lower costs per firing cycle.